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I have a natural gas boiler, with cast iron radiators. This is a hot water system, closed loop system.

Just replaced seized bleed valves on 2 cast iron radiators: 1 - Drained the system. 2- Replaced the valves. 3- Refilled system, purged air. Made no other changes to the system except replacing the 2 valves. PRV and expansion tank are 2 years old.

Prior to the above, system pressure read 12 PSI (cold). Now it reads 17 PSI (cold). Why? What could have caused this change?

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  • Most boiler setups have a prv that allows some water to enter the system if the pressure goes under the prv setting - you have a valve that you keep off after filling? Even in a "closed" system typically the plumbers have always had me keep the valve partially open in case any water escapes. Sep 7, 2023 at 15:34

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  • Either it was re-filled to 17 PSI.
  • Or, the cold water from the street/well (probably 50 °F/10°C or so) is considerably colder than the "cold system" temperature in the house (ballparking 68°F/20°C or more) in early September, so it's expanded slightly.
  • Or the dissolved air (in the new fill water) that hasn't been purged yet has come out of solution and raised the pressure. Whenever you add water, new air comes into the system and needs to be purged. It will not, generally, all come out when you first refill, so it can not, generally, all be purged right then and there. Some can be, the rest needs to be dealt with in the following days or weeks.

There's nothing inherently wrong with being at 17 PSI cold, if the system does not overpressure when hot.

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